Heart pumps are typically used in the later stages of heart disease or after trauma to the heart, when the heart itself is too weak or otherwise incapable of creating sufficient blood pressure and blood circulation to satisfy body function.
Various heart pumps are already in use for the purpose of augmenting or replacing the blood pumping action of damaged or diseased hearts. Heart pumps are commonly used in three situations: (1) for acute support during cardio-pulmonary operations; (2) for short-term support while awaiting recovery of the heart from surgery; or (3) as a bridge to keep a patient alive while awaiting heart transplantation. The pumps may be designed to provide at least one of right or left ventricular assist, although left ventricular assist is the most common application in that it is far more common for the left ventricle to become diseased or damaged than it is for the right ventricle.
Intravascular pumps comprise miniaturized pumps capable of being percutaneously or surgically introduced into the vascular system of a patient, typically to provide left or right heart support, or even total heart support. Various types of intravascular pumps include radial flow centrifugal pumps and axial flow pumps. One form of axial flow heart pump is disclosed in co-pending, commonly assigned U.S. patent application Ser. No. 12/322,746 (“the '746 application”), the disclosure of which is hereby incorporated by reference herein. An axial flow pump according to certain embodiments of the '746 application uses magnetic or electromagnetic forces, for example, to power a magnetic rotor placed within a flow path of blood moving into or out of the heart. An electromagnet, or stator, is positioned around the outside of a tubular casing containing the flow path, whereas the rotor is disposed inside the casing. The axis of the rotor is coincident with the axis of the casing.
The rotor is magnetic. The stator typically is a set of electrically conductive coils. The rotor is energized by a power source with alternating currents through the coils to create a rotating magnetic field. That is, the field is directed transverse to the axis of the tubular casing, and the direction of the field rotates about the axis of the casing. As the field rotates, the rotor spins about its axis. The rotor is configured with vanes which impel the blood in a downstream direction along the axis as the rotor turns in a forward direction of rotation. The circumferential surface of the rotor includes hydrodynamic bearing surfaces. As the rotor turns, these surfaces generate radial forces which hold the rotor radially centered in the casing and out of contact with the wall of the casing. The magnetic field of the stator tends to keep the rotor at a fixed axial position, in alignment with the stator. This effect is commonly referred to as the “axial stiffness” of the stator. The power source may be implanted somewhere within the body of the patient or may be external to the patient, as is known in the art.
Axial flow cardiac pumps are efficient, compact and reliable. However, the forces associated with axial flow cardiac pumps can present problems. As the rotor impels the blood in the downstream direction, the blood applies thrust forces to the rotor. These thrust forces urge the rotor in the upstream direction. Moreover, the static pressure of the blood downstream from the rotor is greater than the static pressure of the blood upstream from the rotor. The pressure differential also urges the rotor in the upstream direction along the axis. Under certain flow conditions, especially at lower flow volumes and/or higher rotational speeds, the axial stiffness of the stator may not be capable of counteracting the axial forces on the rotor. In this case, the rotor may even contact a mechanical stop, such as a narrowed portion of the casing, which is provided as a backup or emergency restraint. Such contact can cause wear on the rotor and casing, which in turn can shorten the life of the pump and cause other undesirable effects.